feat: add json_body auth method type

[matthiastjong]

Summary

• New json_body auth method type that merges stored JSON credentials into the HTTP request body at the gateway
• Stored fields override agent-supplied fields, keeping secrets safe from the agent
• Use case: APIs like GoCardless Bank Account Data that require credentials in the POST body

Changes

• Service: json_body added to VALID_TYPES + credential hint shows key names
• Gateway: Body merge logic — parses stored JSON + agent body, merges with { ...agentBody, ...storedFields }
• UI: Raw JSON textarea in auth method form
• Form parsing: JSON object validation on create/edit
• Tests: 3 integration tests (basic merge, stored-wins override, empty body)

Test plan

• 268/268 tests passing (32 files)
• 3 new gateway integration tests cover: basic merge, conflict resolution, empty body
• Manual: create target, add json_body auth method, proxy POST request

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